Gas analyzer



Feb. 19, 1957 w H. How; GAS ANALYZER Filed Aug. 31, 1954 Jill xo4momwh.40m-P200 INVENTOR. wpLFRi-:D H. HovvE mm OEEUH Y N Nw Nm mm ww l QH? w-\10N mpom M 5450 ooo N9 l5 l 65u50 mainz/4l I m95? 526m mozmzn /a /8 VIIIAGENT United States Patent GAS ANALYZER Wilfred-H. Howe, Sharon, Mass.,assigner to The Fox .boro;Company,. Foxboro, Mass.,.a corporation ofMassachusetts .Application August 31, .1954, serial No. 455,230

9 Claims. (.Cl. .Z3-255) 'This invention-relates to apparatus for theanalysis of mixtures of .combustible gases, especiallymixtures in suchproportions below the lower explosive limit ortheiipper explosive limitsuch that combustion Yin the mixture is not self-sustaining. 4Itparticularly relates to electrical bridge apparatus which relies on thefamiliar principle that induced combustion of such mixtures `V will-occur in a limited area immediately adjacent to alilame'nt which isheated to a temperature above the ignition temperature of thecombustible gas. The heat generated by'such induced combustion isrelated to the composition of lthe mixture, and such apparatus providesa measurement .of the heat generated as a measurement of lthecomposition of the mixture under test.

An .important use for such apparatus is in chemical operations tomeasure mixtures of combustible gasand air'intorderto maintain thesemixtures at an optimum composition .for the particular chemicaloperatioiijnd still have asafe margin below :the lower explosion limitofthe mixture.

VConventional apparatus operating on this `principle employs a directcurrent Wheatstone bridgelmeasuring circuit including in one arm avheated ilarneiitdn 'cf tact with ythe gaseous mixture tor be measuld.'Thisiilamentreceives heat from two sources. 1F, st, ajcons'ta'nt directcurrent potential is lapplied to the'bridge sufifc" nt tol maintainarcurrent through the iilam'entlwhich w,ill heat it 'to a temperature,greater than the ignition peratureof the 'combustible gas, evenwithnofcombustible tgas present. ,'Second, induced combustion k:of .thegaseous mixture "occurring in `the area immediately 'ajacent Lto ytheheated .lament further heats v the "lf ment to 4an extent related-.tothe composition of theirnjixtlyire. In vthis' conventional arrangementythe "Wheatstone :bridge measurement of theresistance ofthe lfila/nteutvthus gives a measure ofthe composition of the combustible 'mixture. .fItis common practice to :include afsecoiidfdupli'- cate *filament inasecondari-m of" the 'Whea'tstoneb This duplicate `iila-ment 'resistor'is' presumablyffiected by ambient temperature, 'by minorehan'ges in thesupply lvoltage and lby aging e'eetsyexa'ctly the samelafsh themeasuring lament is vaffected so that 'these conditions do lnotdisturb'the relationship between thecomposition "of fthe `gaseousmixture and fthe resistance 'readings l'obtainable from the bridge.

c 1' Conventional prior artapparatusof this typetemploys a platinumiilament -asa measuring element lresistor. This filament deterioratesappreciably whenimainta'ined at the inecessary temperature justabove'the ignition-'temperature forcombustible gas. Evaporationandtotheivdeterioration of the. measuring .lilament increasesexponentiallyias'the .temperature of ithis lament is'in'crea'seddue toztlre :heat from Vthe combustible gas Isurrounding fit.' Sincethereference filament is vnot heated by the lgas com- 'bustion,ldeterioration tof the measuring iand reference-'fila-4 ments occurs atdifferent rates, :particularly l'in lthe' measurement of gaseouslmixturesfapproaching the ilower explosion limit. Thisdeteriorationchangesthe :resistaA rice , 2 s ance "characteristics ofthe ilaments andy thus not only upsets thc, balance of they bridgeY but-lS'c upfctc .thc equalityiof characteristics between rneas'uring'andreiferencelamen'ts.

.This diiculty bcccmcs cxtreme in scmcgcSq-lcnding operations where itisimportant to 'maintain a mixture at a value :as closer as rpossible 'tothe explosion while 'maintaining adequate vsafety .against VVactualexplo? sfive mixtures. `In certainjprocesses the yield o f u sejfulproduct increases rapidly as the composition approaches the explosionlimit. lSince'the degree to which'the ex. plosjion limit can vsafely beapproached Yis dependent largcly Vcron thc accuracy cf'thc measurementalid'thc stability of the vcontrol of ythe compos ition,V it becomes ofii amount impcrtancc tc p'rcvldc an accurate; ,Sta'blcY ',fl'cliablemeasurement. In .the 'conventional constant v potential directcurrent'bridge measuring system, ,the` 'staf bilitygand reliability ofthe measurementjfalls "short of what is 'desired in yspite of frequentextensive and costly maintenance procedures.

The present invention overcomes the A,prior artdiiriculties by measuringthe heat of combustion without'the usual temperature risein themeasuring fiilantept andby providing a combination yalternatingcurrent-direct V,current bridge circuit arrangement.

currentibjridge With'a measuring resi t by alternating current, with acontrol feedba li Yfrom the output ofthe 'bridge to control thealternating vcurrent and ,therebyjtov vcontrol its .heatingyefyecg as am'ns' of i'rramtaining abridgebalance condition, and with ay directcurrent lfunction and an alternating 'current f unction'fe'ssentillyisolated with respect to each other, l

Thus the present invention provides a `novel gas analyze ing systemwherein the electrical resistance andhencethe temperature of'theme'asuring filament is vin tainedlat a' substantially constant Valuejust "suliicient to assure'finduced combustion of the gaseous mixture..Thisrprovides fonminimm deterioration of the jrneasrurihng vtilam t.VFurthermore, siiiee both the vmeasuring an iilaments are heldatsubstantially equal ,rr i retain accuracy of ybalance characteristicsperiods. This both increases the lreliability l measurement yand reducesthe maintenance problems'gin checking and rebalancing the/measuringcircuit/,topgamp'ensate f'fo'rrapid deterioration 'ofy the "measuringlilarne'nt and also very considerably lengtheus thcilfclful life/@f thc'mccsics lamcht, thereby fcducia the .cccltly rcplacenre'nt'andrecalibration at frequent interyalsrequired with' the conventionalsystem. n f i l 'Y .An embodiment of A"the present invention,which ywill be described in detail he'r'elinafter, `utilizesa direct r"currentWheatfstone'bridge 'for the measurement ofthe relative Vresistance vofthe 'measuring and yreference filaments in a fr'nariner 4resemblingconventional practice. However,.current limiting resistanc inthedirect'curretit supply-circuit limitsthe direct current tlow te vgarelatively small value. In a preferred embodiment the directeur-'rentbridgeccurrent provides roughelyy one percent ofthe total filamentvheat1ener gy. Separate alternatingfcurrent heater'po'wer circuits areused to supply Velectrical energy tol heat thel filaments .as a means-of Y'providinga constant current supply Yto `.the .reference filamentand fa variable ,current supply toV the measurement,iilament Thecurrentsupply to the measurement iilament is constantly and vautomaticallycontrolled so ,that the heatpro- -duced'tvhereby is reduced as the heatof induced combustion -of the gas increases, to the" endthatthe,measure-` inentiilament temperature yremains substantiallyconstant.

heat energy supplied to the measuring element byinduced combustion ofthe gas under test. This difference is thus a measurement of thecomposition of the measured gaseous mixture. A suitable measuringinstrument circuitmeasures the difference in alternating currentsupplied to the measuring and reference fila ments. This instrument iscalibrated directly in terms of composition of the mixture.

A particular feature of this invention is the structure and arrangementby means of which both alternating current and direct current ow isprovided in the measuring and reference filaments, with each performingits separate function without appreciable interference with the other.The direct current is prevented from circulating in the-alternatingcurrent filament heating circuits. The alternating current issubstantially blocked out of the direct current -bridge supply. Finally,the circuit is designed with an instrument formeasuring the directcurrent balance ofthe bridge with this instrument substantiallyunaffected by the alternating current applied to the system. Similarly,an alternating current instrument is provided which measures therelationship between measuring iilament current and reference filamentcurrent, and thereby the composition of the mixture, with this latterinstrument completely independent of the direct current function in thebridge.

It is therefore an object of this invention to provide a new andimproved gas analysis device.

Other objects and advantages of this invention will be in part apparentand in part pointed out hereinafter. The objects and advantages of thisinvention may be appreciated by reference to the accompanying drawing.

The drawing is a schematic illustration of a hot wire gasanalyzer systemembodying this invention, with the `system shown partly in specificelectrical circuitry and partly in block diagram showings of standard orcommon units, power supplies, amplifiers, etc.

The drawing and the following description relate to a specificillustrative embodiment of a hot wire gas analyzer according to thisinvention. This embodiment is 'based on an all resistance type ofWheatstone bridge 10 with arms 1, 2, 3, and 4, with arm 3 containing ameasuring hot wire resistor filament M at which the gas under test isburned.

The bridge 10 is operated on direct current from a direct current source11 through leads 12 and 13 to opposing corners 14 and 15 of the bridge,in the usual form of Wheatstone bridge excitation. As will be detailedhereinafter, portions of the bridge, i. e. arms 3 and 4, are alsoexcited by alternating currents for resistor heating purposes. In orderfor the direct current function of the bridge to be usable, circuitprovisions must be made for the compatible operation of the alternatingcurrent and direct current systems. These provisions include, in thedirect current bridge leads 12 and 13 from the Adirect current source11, 500 ohm fixed resistors (16) (17). The direct current source isfloating with respect to ground in order not to interfere with theoperation of the bridge, which is grounded at a specific point 18,namely, at the junction of theA bridge arms 3 and 4.

About 100 volts are applied by the direct current source to the two 500ohm fixed resistors 16 and'17 in the bridge leads 12 and 13, and thebridge is thus supplied with about 100 ma. of direct current. Thealternating current in the -bridge is supplied at l2 volts and` 1000cycles and at about one ampere of heating current, so that the.1 ma. ofdirect current has no appreciable 'effect on the alternating currentfunction. As a further provision for making the alternating current anddirect current operations compatible, the bridge arms 1 and 2 areprovided with 500 ohm fixed resistors 19 and 20, and the unbalanceoutput of the bridge is taken from the junction point 21 of the bridgearms 1 and 2. Thus.

4 the direct current function of the bridge is not undesirably affectedby the alternating current in the bridge.

In the bridge 10, arm 3 is the measurement arm, and is made up of aseries arrangement of two one ohm resistors, M and m with resistor madjacent the bridge ground point 18, and resistor M adjacent the bridgearm 2. These low value resistors provide a path of low impedance to thealternating current, and also facilitate the measurement of current inthese resistors as a factor in the operation of the gas analyzer. Theresistor M is the measuring hot wire resistor at which the gesundertestis burned. This measuring resistor M is selected to have a highcoefficient with respect to temperature, using platinum for example, inorder to provide good response in the form of tendency to resistancechange andv consequent bridge unbalance when the heat of combustion ofthe gas under test is applied to the resistor Mr Further, the measuringresistor M is indicated as being` contained in a combustion chamber 22which is so arranged that a gas sample can be continuously drawn orpushed through it.

The resistor m is a standard resistor selected to have a low coefficientwith respect to temperature, in order to provide, for currentmeasurement purposes, an unchanging vehicle for the alternating currentchanges which occur in the measuring resistor M, as will be seenhereinafter, as a result of the application thereto of the heat ofcombustion of the gas under test.

Thus, gas combustion heat results in changes in the alternating currentvalues in the measuring resistor M, while measurement of such currentValues may be made in the adjacent unchanging standard resistor m.

The bridge arm 4 is a duplicate of the bridge arm 3. Two one ohmresistors R and r are arranged in series in this arm. The resistor R isused as a reference resistor; it is adjacent the bridge arm 1; and it iscontained in a chamber 23 which is sealed, but otherwise is comparableto the combustion chamber 22 of the measuring resistor M.

These chambers, 22 and 23, may be connected by a passage (not shown) sothat the same atmosphere of burned gas may surround both the measuringresistor M and the reference resistor R. In this fashion thetemperatures Yof resistors M and R may be closely controlled withrespect to each other since thermal conI ductivity is the same in eachof the resistor ambiencies. In such an arrangement the heat of gascombustion which is applied to the reference resistor R is negligible.

The resistor r, as the second resistor in the series of the bridge arm4, is a standard resistor, like the resistor m of the bridge arm 3, andthe resistor ris adjacent the bridge ground point 18.

The bridge arms 3 and 4 are provided with `1000 cycle `alternatingcurrent for the purpose of heating the measuring and reference resistorsM and R. The alternating current in the reference resistor R is providedas a fixed component, while the alternating current in the measuringresistor M is provided as a controllable, variable component.

` The source of the alternating current which is applied to the bridge10 is indicated at the right of the drawing at 24, in block outlineform. This source may be used to provide all the alternating current inthe whole analyzer system, as 'a means of tieing all the circuitstogether to prevent inaccuracies due to supply vari; ations. Forexample, the frequency f and phasing of alternating currents fromdifferent sources may vary undesirably, reducing the accuracy of the gasanalyzer. Further, variations in hot wire resistance due to fluctuationsin heating current are cancelled by similarly applying heating currentto the reference resistance. Consequently, the effect of suchfluctuations on the output of the'bridge is negligible.

The 1000`cycle alternating current source 24 is an oscillatori which,through leads 25 and 26, provides activating alternating current zto aadistribution'ftransformer-'27. From'the distribution transformer. 21,leads 28 "and `29 carry the alternating current'tothe .bridge 10, .by-Way-of a pair of :amplifiers 30 and $31. These amplifiers are poweredfrom'acommon fpower supply 32. Thegan of the amplier issfixedgiand itsoutputis fapplied across 'the bridge arm^4tthroughatrans former 33. Thesecondary 'zof theitransformer-33 is part of a circuit which includesthe bridge. arm'4 and consequently 'the reference resistor .lRand `thestandard resistorv r. As a pa'rt of .thisfreference resistor circuit a.connection from .the `bridge `'ground point 18.10 the secondaryofthetransformer contains adirect current blocking condenser :34.'an'd `anassciatedfchoke 35 rfor relating thealternating .current toIthe'jcon'denser 34.

.The -variable :gain amplifier lZilfapplies alternating currentlacrossthe bridgefarm'.'3,2and therefore' to the measuring resistor. IMand-fits associated* standard resistor m, through f`a ftra'nsformer 36.The measuring 'resistor bridge arm '3.is partiof an alternatingcurr-cutvcircuiteincludin'g the secondary of the transformer n36 and a blockingcondenser 37 Vwith an associated choke '38, inman ner and purposecomparable to vthe-alternating current circuit .of the referenceresistor r.

The pattern of alternating currentadirect "zcurrent voperationcompatibility `in the bridge`fisthus-complete. Undesirable 'ialternating current eifects '1 arevr kept from 'the direct vcurrentinput 'by the SOOohm'resi'stors 16an`d"17, and fromthe direct current-output'byfthe i500V hmre si'stors 19fa'n'd 'ZG-inthebridge arms 'f1and'2. jIn the alternating 'currentsectionof `the"bridg`e, the directcurrent component is conned to thebridgeand kept'from leaking lofintoalternatingcurrentl circuits, Vby the blocking condensers 34 and37inlthelalt'ernatingcurrent circuits'which include 'the `A`bridge VarmsVl4 and 3" respectively.

The alternating current f components as Asupplied tto the bridged() 'areLalsof.compatible with eachiother, that is,'-they Vare essentiallyisolated f'rom each "other, :since the transformer'secondary "circuitsprovide complete, lov/*impedance circuits which are 4essentiallyindependent of each other. 1

ln view of the foregoing, therefore, the referencere- Ysi'stor R may beyconsidered to'be initially ``heated to a predetermined temperaturer'bythejalternating current component therein. The resistor 'Risi heated,'for practical purposes, `only by the 4alternating current therein. Theapplication of'the output`of'the`fxedjgainarnplitierA 30 to' the lbridgearmy #presumably vprovides a' fixed value alternating current rin 'the'reference 'resistorlSR, with the' result thatjtheresistor. R'ispresumably heated to :aA predeterminedxed 'temperature andy is.'presumably provided Hwith` a predetermined, fixed I resistance value.While the vstandard resistor r' is subjected to`the same alternating`current,"its co'eicient with respect to temperature islow and itstemperature. and resistance values remain "essentially unchanged.

The yariable'gain `amplifier 31 is initially` adjustedin its applicationof' alternating current to `tlie'b'ridge arm.

3 until the bridge 1) is balanced, usually'because .the measuringresistor` M yis provided lwith. a Vresi'stance'v'alue which is balancedagainst the resistance Value yofthe reference resistorlR, as heated'byAalternating current from, the xedgainarnplier 30.

Thus the'bridge'ld is in condition for operation.y It is excited withdirect current, portions .3.;.and 4..'of...it arefh'ea'ted byalternating"currennandthe-bridge. is balanced bothfwith respect .tovdirect currentfand .with 'respect to .alternating current. n

measuring resistor .combustion chamber 22and the heatezlmeasuring`resistor-1M 'atleast' initiates combustionin tl'iezgaszsample. Y Themeasuring resistor; Mzrnay be specially. treatdltofprouide fcatalyticcombustion'fof Sthewgas; thez v'gasv 'combustionlmay beridu'e mainly'tovthe Atthis -point..ar gas sample is introducedl intothe ifromflthe-bridge output lpointhZl 'to ground,y that is,1to

the "fbridgeground 'point ylill. 'This output fis applied through a lead139 and 'an alternating current filter l40 to a unit i410 which isfa'direct-current voltage -d'etector and amplifier. The output of theu'nitl40' 4is vappliedlto the #variable vgain 'amplifier r431 las -a'control feedback whichvariesthe alternating current inthe l-bridgelarm-3 in 'faccordance `with the bridge `unbalance "condition,

lThe operation' ofthe; bridge 10, therefore, may besuma rnarized asfollows: The bridge is activated by direct current, and'fthereferenceand measuring resistors R and Ml are initially '-heated' by alternatingcurrent in f-fsuch a `mannenthatthe"bridge is'rbalanced. A sample-'ofthe gas funderetest is 'exposed-to 'the heated measuring resistor M, andthe gas sample is burned. The I.heatof combustion Aof zthefffgas Tisapplied' to 'the measuring resistorMs'withtherresult that the resistance`value ofthe 'measuringjresistor M is increased. This/increasedresistanceunbalances the'bridge and through-the detector andamplifier-Iunit740',l -brings Ithe control 'feedback'intoOperatOn'toreduCc the -gain of vthe `variableffga-in arnplifier '31,with fthe v-zresultl 'that 'the :alternating current finthe#lmeasuringresistor M is reduced.Y Consequentlyfthe heating? effectof Vthe alternating currentvis reduced and the -resistance value.v of--the measuring resistor`M is reduced toits initial value, and thebridgel isrebalanced. he effect off'theabove .faction von the measuringresister 'M\is'to'fsubstitute? heat from the gas combustion for heatfrom the alternating current, with Athe resulting currentvreductionavailable as afactor in thevrneasurementofthe-.degre'e ofcombustibility of the vgas-urider test. lnftheoperation'of thebridgeAEdi, therefore, 'the temperatures :and` resistance values of thereference and measuringaresistorsR and M are maintained essentiallyequal, with the Variant/being the alternating current-value in themeasuring resistor Vi. f

Various-conditions inthe bridge are representative of the amount of.heat applied to the-ineasuring'resistorfM as Aa resultofthecombustionof the gas underltest. The tendency toward-bridge unbalance is one suchcondition, an'clthe resultant change in thehea'tin'g alternatingcu-rrent in the fvrneasu'rin'g resistor Yisv another, IHowever, thefactor whichis used as ameasurableicondition ini-representation ofv the'degree of'combustibility of the =gas under test, i.-` c inrepresentation of 'the heat .of combustion-of the gas under test, is theratio between the'alternatingcurrentin the reference'resi'stori?` andthe alternating current in the measuring'resistor M. "For practicalpurposes.y

circuit 3S. VThe reference and measuring resistorsRV andV M are notincluded -in this measuring circuit.

The pseudo'bridge circuit-comprises the standardv resisters rand m astwo adjacent bridge arms withthe ground point 18 therebetween, a pair of100,000 ohm resistors connected to the midpoints of the measuringjan'dreference bridge arms 3 and 4, between the standard resistor and Athemeasuring or reference resistor `-as vthe k case may be,z and''a'50,000ohm resistor4 connecting the outerends ofthe two'1`00,000'ohm resistors.A- connec- `tionV slider 41 is provided on the 50,000 ohm resistor inthe mid-point area thereof, with an output lead 42 extending from theconnection slider 41.V

The pseudo bridge 38 is not a true bridge because there is noappreciable variation of any of the resistor values. Instead, thecurrent values are made to difer in different arms of this bridge, andthe ratio between these currents is measured in potentiometric fashion.The potentials at the connections between the standard resistors r and mand their respectively associated reference and measuring resistors Rand M are a measure of the 1000 cycle current values in the referenceand measuring resistors. Further, the potential on the slider connection41 is a measure of the difference in potentialacross the standardresistors r and m. Since this slider connection difference voltage isproportional to the ratio between the current in the reference standardresistor r and the current in the measuring standard resistor m, theslider connection (41) potential is a measure of the unbalance betweenthe measuring and the reference resistors M and R, and of the degree ofcombustibility of the gas under test.

The potential at the slider connection 41 is used to provide a record ofthe degree of combustibility of the gas under test, by applying thispotential to a recording system which is shown in the drawing asenclosed in a dotted line box 43.

The lead 42 from the slider connection 41 to the recording system 43 isprovided with a blocking condenser 44 to prevent any direct current fromreaching the recording system, and a 1000 cycle tuned filter 45 isprovided between the lead 42 and ground. This filter has an impedance to1000 cycle alternating current in the order of one-half megohm, withvery much lower impedance to all other frequencies. As a result, thelter forms an effective by-pass to spurious noise frequencies. Avariable coupling condenser 46 is located in the slider connection lead42 beyond the tuned filter 4S, as an imepdance for controlling theeffectiveness of the hot wire bridge output voltage, and the sensitivityof the recording system 43.

The recording system 43 may be considered as a servo amplifier with abalanceable recorder bridge 47 therein. The output of the recorderbridge 47, from the bridge unbalance point 48, is connected to thepseudo-bridge output lead 42, with the resultant voltage fed through anunbalance voltage amplifier 49 to the rest of the rebalancing servonetwork.

The remainder of the servo network comprises an unbalance voltagedetector 50, a power amplifier 51 and a pen drive motor 52, with the pendrive motor producing a mechanical movement. A vmechanical connection 53is used to apply the pen drive movement to adjust a variable,rebalancing condenser 54 in the recorder bridge 47. I

This condenser adjustment movement is applied, through a mechanicalconnection 55, to a recorder pen 56 as a means of moving the pen 56generally radially over a rotating chart 57.

The recorder bridge 47 is formed with the upper arms thereof each havinga variable coindenser therein, one of which is the previously mentionedrebalancing condenser 54; and with the lower arms thereof each providedwith a fixed resistor 58, 59. The output connection 48 of the recorderbridge 47 is between the two variable` condensers, and the groundconnection 60 of the recorder bridge 47 is between the two fixedresistors 58, 59. The recorder bridge 47 Ais excited from thedistribution transfonner 27, and the servo network is provided With 1000cycle alternating current from the oscillator 24 through leads 61, 62 tothe unbalance voltage detector 50.

The gas analyzer described herein therefore operates to supply heating,alternating current of a suitable fixed value to the reference resistorR, and heating, alternating current of a variabile value to themeasuring resistor M.

This current to the measuring resistor M is controlled at a value suchthat the resistance, and hence, presumably, the temperature of themeasuring resistor M, is the same as that of the reference resistor R.The, recording system 43 provides a continuous record of the relation ofthe alternating currents in the reference and measuring resistors R andM. The difference between these currents is presumably due to the heatsupplied to the measuring resistor R by the combustion of the gas undertest, and hence the record shows the approach to the explosive limit ofthe gas. The analyzer depends on a combined direct current and 1000cycle alternating current operation of the hot wire bridge 10 and itsassociated structure. The direct current bridge 10 circuit measures theresistance of the reference and measuring resistors R and M. Unbalanceof this bridge 10, operating through the direct current amplifier 40',provides a direct current controlto the 1000 cycle variable gainamplifier 31 and consequently to the alternating current in themeasuring resistor M. This control operates on the alternating currentsupply from the variable gain amplifier 31 to hold this supply to such avalue that the resistance of the measuring resistor M is maintainedequal to the resistance of the reference resistor R, which is heated bythe constant 1000 cycle alternating current supply from the fixed gainamplifier 30.` A continuous record of the ratio of the two alternatingcurrent heating currents in the measuring and reference resistors M andR in effect represents the reduced electrical heating of the measuringresistor M as its heating current is reduced, and thereby, the amount ofheat supplied by combustion of the gas sample.

Thisinvention therefore provides a new and improved gas analysis deviceof the hot wire type.

As many embodiments may be made of the above invention, and as changesmay be made in the embodiments set forth above, without departing fromthe scope of the invention, it is to be understood that al1 matterhereinbefore set forth or shown in the accompanying drawing is to beinterpreted as illustrative only and not in a limiting sense.

I claim:

1. In an electrical bridge gas combustibility measuring system, incombination, a direct current excited electrical bridge, a referenceresistance in one arm of said bridge, a measuring resistance in anotherarm of said bridge, means for associating gas with said measuringresistance for combustion by heat from said measuring resistance, afirst alternating current supply means essenltially exclusivelyoperatively associated with said reference resistance arm for producinga predetermined set value of alternating current in said referenceresistance, a second alternating current supply means essentiallyexclusively operatively associated with said measuring resistance armfor producing and varying a gas combustion temperature in said measuringresistance, means operatively connecting the output of said directcurrent bridge to said second alternating current supply means, wherebya predetermined resistance value relation is maintained between saidmeasuring resistance and said reference resistance, and means for makingthe direct current function in said bridge compatible with respect tosaid alternating currentcomponents.

2. A hot wire electricalrbridge circuit gas analyzer comprising, incombination, a direct current excited bridge, a reference resistor and ameasuring resistor in different portions of said bridge, means forheating said reference resistor by applying a fixed alternating currentessentially exclusively vto said reference resistor portion of saidbridge, means for heating said measuring resistor by applying a variablealternating current essentially exclusively to said measuring resistorportion of said bridge, means for burning a gas adjacent said measuringresistor to tend to change the balance condition of said bridge bytending to change the resistance value of saisimeasurina'-lr-esstor-rand; means. fory vil-frias. Said vari ble altriian g current to.ch ange, the heatinggcle@ th ,eofv 1 accordance withimbalance conditiorrsmof,` said fgeandfy nrep'resentation;ofrthe degreeof;y combusti.-y blity oflfsaid. gas, saidmeansfforvarying saidvariablealternating current including an electrical connection,

fromtle outputfof said bridge tosaid,variablevalternatingwcurrentfaplyinglmean's.. i

3,v hotfw'ireelectrical gas. analyzer wherein-thetemr pe'raturejotffthemeasuring hot wireresults. fromthe addi-4 tivejeltectof the heatl'generated' by electric current flow as, saidftemperaturebeingmaintainedl 'ntfrallj constantlby, automatically. increasing, orVde;`

crea lng; trieeiearr'iearneaung current, as. the heat ror l co us 1oofthe gas decreases orincreases, comprising iniV combination,vafdirect'current` system with` a bridge maintained' substantiallyconstant thereby" maintaining,

said measuring hot wire at' a substantially `corrstantpre determinedtemperature suicient to assure combustion of the measured gas, circuitprovisions for the compatible operation of the functions of saidalternating and direct currents, including predetermined and sufcientalternating current impedance in the direct current power supply, thebalancing arms of the direct current bridge, and the direct currentinput to the control circuit, to limit the alternating current flow inthese circuit elements,

and predetermined and suilicient direct current imped-V ance in thetixed and variable alternating current sources and in the alternatingcurrent comparison circuit to limit the direct current therein, meansfor comparing the variable alternating current in said measuring hotwire and the substantially fixed alternating current in said referencehot wire, and means operable from said comparison of the variable andsubstantially fixed alternating currents to provide a measurement of theheat of combustion and hence of the composition of the measured gas forthe operation of indicating, recording and control means singly and inany combination thereof.

4. A lhot wire electrical gas analyzer comprising, a Wheatstone-bridge,a direct current source connected to energize said bridge, a referenceresistor in one arm of said bridge, a measuring resistor in anotherk armof said bridge for receiving heat of combustion of a gas as a means ofdetermining the degree of combustibility of said gas, a controlledalternating current supply network arranged to provide constant heatingexcitation essentially exclusively to said reference resistor arm and toprovide variable heating excitation essentially exclusively to saidmeasuring resistor arm, a feedback connection from the output of thedirect current bridge to the controlled alternatng current supply toprovide a control of alternating current supply to the measuringresistor as a means of automatically maintaining direct current bridgebalance and consequently essential equality of resistances in themeasuring and reference` resistors, and means for essentially isolatingthe direct current function in said bridge with respect to saidalternating current component whereby change in said controlledalternating current is available as a gas combustibility measurementsignal forr the operation of recording, indicating and control means,singly and in any combination thereof. Y

d from heat frorninducedcombustion v i@ l5?, Inahot wireeleetricalbridge circuit gas analyzer,- whereip afgasrisburnedat thechot wire withthe` resultthat the heat ofY sue-hcombustionv tends to. change; t heelectrical resistance ofl the' hotfwire toV producey a bridgev'reslgrect to {said-,variable alternating current, andl an alter.-

natjng current; pseudo bridge includingA an alternating.

current portiono said direct current bridge for? potenti-v ometricallymeasuring alternating currentchange in saidbridge.l Y

6;. In a: gas analyzer; wherein the electrical condition of.4 atnannten-bla electrical' bridge is; inauencedby inny ofycombustiomofasample offthefgas undertest in representationofthe degreeocombustibility` of the gas-under testg'adirecta= current excitedl andbalanceable bridge, al reference-resisten inone arm ot said bridge, ameasuring resist rinanotherjarmof Saidl bridge for responsiveassociationwithsaidfheat ocombustion of said gass'ample,meanaforgapplyingaxedvalue alternating `current clicclyA tosaidreference-resistor bridge arm as ,n y v f tdcstablishga resistancevalue in said referenc resior,- means forapplyi-ng-a variable valuealternatingfjcurrent etectivelyexclusively to said measuring;resistorlbridge, arm as,a;heating; action to establish a resistance value in saidmeasuring resistor'as a balancing action with respect to said referenceresistor, control feedback means from the output of said bridge forvarying said variable'alternating current with direct current inaccordance with unbalance conditions of said bridge, whereby a tendencyto change the balance condition of said bridge isautomaticallycounteracted by a change in said variable alternating current asrepresentative of the degree of combustibility of the gas under test andwith a consequent change in the resistance value producing heatingeitect of the variable alternating current, whereby said variablealternating current change is available as a gas combustibilitymeasurement signal for the operation of recording, indicating, andcontrol means, singly and in any combination thereof.

7. A hot wire electrical bridge circuit gas analyzer, wherein a gas isburned by catalytic combustion at the hot wire with the result that theheat of such combustion tends to change the electrical resistance of thehot wire to produce a bridge balance condition changing effect whichisameasure of the degree of combustibility of the gasunder test,comprising, in combination, a direct current operated bridge,` ameasuring hot wire resistor and a standard resistor in series therewithin one arm of said Y bridge, with said arm heated by variableralternating current, a control feedback from the output of said bridgeto said variable alternating current, means for essentially isolatingthe direct current function in said bridge circuit lwith respect to saidvariable alternating current, and a ence resistance arm, means forassociating gas withsaid l measuring resistance for burning by catalyticcombustion at said measuring resistance, a first alternating currentsupply means for essentially exclusively heating said refl nasales'erencc arm and producing a predetermined set value in said referenceresistance, a second alternating current supply means for essentiallyexclusively heating said measuring arm and for producing and varying agas combustion temperature in said measuring resistance, meansoperatively connecting the output of said direct current bridge to saidsecond alternating current supply means, whereby a predeterminedresistance value relation is maintained between said measuringresistance and said reference resistance, means for essentiallyisolating the direct current function in said bridge with respect tosaid alternating current components, and a pseudo bridge forpotentiometrically measuring the ratio between alternating current insaid measuring resistance arm and alternating current in said referenceresistance arm, said pseudo bridge including said standard resistances.

9. A hot wire electrical bridge circuit gas analyzer, wherein gas isburned by catalytic combustion at the hot wire with'the result that theheat of such combustion tends to change the electrical resistance of thehot wire to produce a bridge balance condition changing effect which isa measure of the degree of combustibility ofthe gas under test,comprising, in combination, a Wheatstone bridge, said bridge having a500 ohm resistor in each of two adjacent arms, a one ohm referenceresistor and a one ohm standard resistor in series in a third arm ofsaid bridge and a one ohm high temperature coeicient measuring resistorand a one ohm low temperature coefficient standard resistor in series inthe fourth arm of said bridge, a 100 volt direct current supply having500 ohm resistance leads to said bridge between each of said 500 ohmarms and their respectively adjacent one ohm resistor arms, said bridgehaving a ground connection between said one ohm resistor arms and anoutput connection betweensaid 500 ohm arms, a gas sample combustionchamber containing said measuring resistor, a controlled 1000 cycle, 12volt, alternating current supply network arranged to provide constantheating excitation to said reference resistor arm and to providevariable heating excitation to said measuring resistor arm, a directcurrent blocking condenser arrangement between said bridge and saidalternating current supply network, a control feedback connection fromthe output of the direct current bridge to the controlled alternatingcurrent supply to provide a control of alternating current supply to themeasuring resistor as a means of automatically maintaining directcurrent bridge balance, and a pseudo bridge for potentiometricallymeasuring the ratio between alternating current in said measuringresistance arm and alternating current in said reference resistance arm,said pseudo bridge including said standard resistors.

References Cited in the tile of this patent UNITED STATES PATENTS2,219,540 Miller Oct. 29, 1940 2,467,856 Rich Apr. 19, 1949 2,617,716Hartline Nov. 11, 1952 2,650,496 Middleton Sept. 1, 1953 OTHERREFERENCES Munch: Ind. and Eng. Chem, vol. 42, No. 2, February 1950,pages 69A and 70A.

1. IN AN ELECTRICAL BRIDGE GAS COMBUSTIBILITY MEASURING SYSTEM, INCOMBINATION, A DIRECT CURRENT EXCITED ELECTRICAL BRIDGE, A REFERENCERESISTANCE IN ONE ARM OF SAID BRIDGE, A MEASURING RESISTANCE IN ANOTHERARM OF SAID BRIDGE, MEANS FOR ASSOCIATING GAS WITH SAID MEASURINGRESISTANCE FOR COMBUSTION BY HEAT FROM SAID MEASURING RESISTANC, A FIRSTALTERNATING CURRENT SUPPLY MEANS ESSENTIALLY EXCLUSIVELY OPERATIVELYASSOCIATED WITH SAID REFERENCE RESISTANCE ARM FOR PRODUCING APREDETERMINED SET VALUE OF ALTERNATING CURRENT IN SAID REFERENCERESISTANCE, A SECOND ALTERNATING CURRENT SUPPLY MEANS ESSENTIALLYEXCLUSIVELY OPERATIVELY ASSOCIATED WITH SAID MEASURING RESISTANCE ARMFOR PRODUCING AND VARYING A GAS COMBUSTION TEMPERATURE IN SAID MEASURINGRESISTANCE, MEANS OPERATIVELY CONNECTING THE OUTPUT OF SAID DIRECTCURRENT BRIDGE TO SAID SECOND ALTERNATING CURRENT SUPPLY MEANS, WHEREBYA PREDETERMINED RESISTANCE VALUE RELATION IS MAINTAINED BETWEEN SAIDMEASURING RESISTANCE AND SAID REFERENCE RESISTANCE, AND MEANS FOR MAKINGTHE DIRECT CURRENT FUNCTION IN SAID BRIDGE COMPATIBLE WITH RESPECT TOSAID ALTERNATING CURRENT COMPONENTS.